Different climatic conditions are produced by the geography
and uneven heating of the Earth. Plant and animal forms that are
characteristic of a particular geographic area with a common climate
constitute biomes. Each biome is characterized by specific climax
communities. All the biomes together form the biosphere.
The various biotic and abiotic factors at play on Earth
result in six major terrestrial biomes. Terrestrial biomes are categorized
according to the types of plants they support. The fundamental characteristics
of each type are described in the list below.
Tropical Rain Forest
Rain forests have the highest rainfall of all biomes (100–180
inches per year), which results in the greatest animal and plant
diversity. Trees form canopies that block sunlight from reaching
the ground. Most animal species live in the canopy, while the forest
floor is inhabited predominantly by insects and saprophytes and
consists of soil low in nutrients. Decomposed products on the forest
floor are washed away or quickly reabsorbed by plants. Tropical
rain forests can be found in Central America, the Amazon basin in
South America, Central Africa, and Southeast Asia.
This biome is characterized by grassland with sparse trees,
with extended dry periods or droughts. Tropical savannas generally
border rain forests and receive a yearly total of 40 to 60 inches
of rainfall. They support large herbivores, such as antelope, zebra,
elephants, and giraffe. Most tropical savannas exist in Africa.
Temperate savannas, such as the Pampas in Argentina and the prairies
east of the Rocky Mountains in the United States, receive only about
10 to 30 inches of rain a year. Grasses and shrubs dominate the
landscape and support insects, birds, smaller burrowing animals,
and larger, hoofed animals such as bison.
Deserts are the driest biome, receiving less than 10 inches
of rain per year. They exhibit radical temperature changes between
day and night. Animals of the desert such as lizards, snakes, birds,
and insects are typically small and have adapted to the dry, hot
climate by being nocturnally active. Plants, such as cactus, have
evolved waxy cuticles, fewer stomata, spiky leaves, and seeds capable
of remaining dormant until sufficient resources are available. Deserts
exist in Asia, Africa, and North America.
Temperate Deciduous Forest
Rainfall in temperate deciduous forests is evenly distributed
throughout the year. The biome has distinct summer and winter seasons.
It has long growing seasons during the summer. In winter, the deciduous
plants drop their leaves and enter a period of dormancy. Beech and
maple dominate in colder variations of this biome, while oak and
hickory are more prevalent where temperatures are warmer. Animals
in deciduous forests are both herbivorous and carnivorous, such
as deer, fox, owl, and squirrel. The forest floor is fertile and
contains fungi and worms. Temperate deciduous forests exist mainly
on the east coast of North America and in central Europe.
The taiga is a forest biome but is colder and receives
less rainfall than deciduous forests. Coniferous (cone-bearing)
trees, especially spruce, dominate the taiga. The trees also have needle-shaped
leaves that help conserve water. Taiga forests sustain birds, small
mammals such as squirrels, large herbivorous mammals such as moose
and elk, and large carnivorous mammals such as wolves and grizzly
bears. Taiga exist mainly in Russian and northern Canada.
This biome is located in the far north and is covered
by ice sheets for the majority of the year. The soil, down to a
few feet, remains permanently frozen, though in the summer, the topsoil
can melt and support a short growing season. Very few plants grow
in the northernmost parts of the tundra, but lichens, mosses, and
grasses occupy some more southern areas. Animals must be well suited
for extreme cold or must migrate. The tundra supports large herbivores
such as reindeer and caribou, large predators such as bear, and
Aquatic biomes account for 70 percent of
the Earth’s surface and contain the majority of plant and animal
life. Aquatic biomes also account for a vast portion of the photosynthesis, and
therefore oxygen production, that occurs on Earth. There are two
types of aquatic biomes, based on the type of water found in each:
marine and freshwater.
Marine biomes refer to the oceans that all connect to
form a single, great body of water. Since water has an immense capacity
to absorb heat with little temperature increase, conditions remain
uniform over these large aquatic bodies. Marine biomes are divided
into three zones: intertidal/littoral, neritic,
The intertidal zone, also called the littoral zone, is
the region where land and water meet. It experiences periodic dryness
with changing tides and is inhabited by algae, sponges, various
mollusks, starfish, and crabs.
The neritic zone extends to 600 feet beneath
the water’s surface and sits on the continental shelf, hundreds
of miles from shores. Algae, crustaceans, and numerous fish inhabit this
The pelagic zone consists of a photic zone (reaching 600 feet
below sea level) and below that an aphotic zone. Light
penetrates the photic zone, which is why it contains photosynthetic
plankton. The photic zone also is home to heterotrophs such as bony
fish, sharks, and whales that prey on these producers as well as
on each other. No light penetrates the aphotic zone, which is a
kind of watery circus of the bizarre, where extreme cold water, darkness,
and high pressure have spurred strange evolutionary paths. The region
is home to some chemosynthetic autotrophs. Other denizens of the
deep are scavengers that feed on dead organic matter falling from
the higher realms and predators who feed on each other.
Freshwater biomes include rivers, lakes, and marshes.
Life here is affected by temperature, salt concentration, light
penetration, depth, and availability of dissolved CO2 and O2. Freshwater
biomes are much smaller than marine biomes, so conditions are less
stable. Organisms that live in these regions must be able to handle
the greater extremes. The very nature of freshwater also demands
special characteristics of the organisms that live within it. In
freshwater environments, the salt concentration within the cell
of an organism is higher than the salt concentration in the water.
A concentration exists between the interior of cells and the exterior
environment: water from the environment is constantly diffusing into
the organism. Organisms in freshwater need homeostatic systems to
maintain proper water balance.